It has been known in the art to employ a lumped “LC” parallel-tuned circuit third harmonic resonator in series with the output terminal of a power amplifier device intermediate the output terminal and a broadcasting antenna. The efficiency of an “LC” resonator is not as great as desired, particularly at VHF and higher frequencies due to circuit losses, stray reactances and undesired resonances, hence, such a “LC” resonator is not practical at these higher frequencies. It is desirable, therefore, to provide an improved third harmonic resonator for use in the output circuit of such an RF amplifying system and located between the RF amplifier device and the output circuit which provides an RF signal to the broadcasting antenna.